1. Field of the Invention
The invention in general relates to the fabrication of ferroelectric materials, and more particularly to annealing a layered superlattice material in an electrical field to provide low fatigue ferroelectric and reliable high dielectric constant integrated circuit devices that are unusually resistant to degradation.
2. Statement of the Problem
Ferroelectric materials can be used in electronic memories. A ferroelectric material is a material with permanent charge dipoles that rise from asymmetries in the crystal structure. The respective positive and negative polarization states can be used to store information by substituting a ferroelectric material for the dielectric capacitor material of a conventional DRAM capacitor circuit. Still, ferroelectric memory densities are limited by the magnitude of polarization that may be obtained from the ferroelectric material. The polarization is the amount of charge associated with the dipolar or free charge in a ferroelectric material. For dipoles, the direction of the polarization is the direction of the dipole. The polarization is equal to the external charge which must be supplied to the material to produce a polarized state from a random state. Once ferroelectric material is polarized, either positive or negative, the material requires approximately twice the amount of external charge to reverse the polarization.
Prior thin film ferroelectric materials typically have high polarization fatigue rates that make them unreliable in long term use because the magnitude of polarization decreases with use. Eventually, the control logic that is coupled with known ferroelectric materials will be unable to read the fatigued polarization state of the materials and, therefore, unable to store or retrieve bits of information.
Ferroelectric layered perovskite-like materials, perhaps better known as layered superlattice materials, are known which have much better resistance to fatigue than prior ferroelectric materials. The term xe2x80x9cperovskite-likexe2x80x9d usually refers to a number of interconnected oxygen octahedra. A primary cell is typically formed of an oxygen octahedral positioned within a cube that is defined by large A-site metals where the oxygen atoms occupy the planar face centers of the cube and a small B-site element occupies the center of the cube.
Prior layered superlattice materials typically have high dielectric constants, and can be used as conventional dielectrics. Even so, too high of a leakage current makes these materials not optimally suited for use as a dense dielectric memory because the charged or uncharged state of the dielectric capacitor circuit must be too frequently detected and refreshed. Leakage current and long term reliability currently present a significant obstacle in research toward further densification of integrated circuit memories.
Hysteresis curves are used to plot the measured charge versus the voltage applied in ferroelectric materials. When the electric field is applied across a ferroelectric material, the polarization can lag behind. When the electric field is applied to ferroelectric material, from positive voltage to negative voltage, the hysteresis plots an elliptical figure if the material exhibits a memory capacity. In a typical hysteresis curve, the voltage applied is measured and plotted on the x-axis with the x-y intersect equaling zero voltage. The measured charge is plotted on the y-axis of the hysteresis curve with the zero measured charge equaling the x-y intersect. As the voltage is cycled from positive to negative voltage, the remnant polarization (Pr) is measured in both the positive cycle and the negative cycle when the voltage equals zero. These polarizations are Pr or xe2x88x92Pr and is the remnant polarization in the ferroelectric material at a zero field value. The Pr and xe2x88x92Pr values ideally have the same magnitude, but the values are most often different in practice. Thus, polarization measured as 2Pr is calculated by adding the absolute values of the actual Pr and xe2x88x92Pr values even though these values may differ in magnitude.
The polarization charge 2Pr is the charge created by switching a capacitor from a state where it is fully polarized in one direction, say the upward vertical direction, to the opposite fully polarized state, which would be the downward vertical direction. An integrated circuit containing a ferroelectric material device, such as a ferroelectric random access memory or a ferroelectric capacitor, stores a charge and, when read by the logic circuit of an electronic instrument or device, returns a zero or a one depending on the polarization state of the ferroelectric material device. When a ferroelectric material device is read millions of times, the polarization value of the ferroelectric material device can change. The greater the 2Pr value the less likely the ferroelectric material device will change polarization states during these numerous read cycles.
It is difficult to find ferroelectric materials that meet commercial requirements. The best materials for integrated ferroelectric devices are switched using a coercive field that can be obtained from conventional integrated circuit operating voltages, i.e., three to five volts. The materials should have a very high polarization, e.g., one exceeding twelve to fifteen xcexcC/cm2 determined as 2Pr, to permit the construction of memories having sufficient densities. While materials with both a suitable coercive field and a high value of 2Pr have been obtained in the laboratory, producing such materials consistently in a commercial environment has been difficult.
As a ferroelectric material device is switched repeatedly, the ferroelectric material fatigues and the plot of 2Pr on the hysteresis curve narrows. This decrease or narrowing of the hysteresis curve is due to the creation of point charge defects arising in the ferroelectric material as a consequence of polarization switching and the associated screening effect of the defects on the applied field, and is generally referred to as xe2x80x9cfatiguexe2x80x9d. This fatigue causes the ferroelectric material to wear out over time due to repeated polarization switching. Generally, commercial materials that have the best 2Pr and coercive field tend to have too much fatigue, and vice-versa. Thus, there remains a need for thin film ferroelectric materials that possess a high 2Pr value, a coercive field that permits switching in the two to five volt range, and are more resistant to fatigue.
Solution
The present invention overcomes the problem of fatigue and too high leakage current by annealing the ferroelectric material or precursor thereof in the presence of an electric field. In another aspect, the invention overcomes the above problems by applying an electric field to an annealed ferroelectric material as it cools. Preferably, this results in polarizing the direction of the ferroelectric materials in the electrical field direction. This insures a greater 2Pr value by increasing the domain of the ferroelectric thin film material, and thereby increasing the 2Pr value. The aligned ferroelectric thin film material also provides a good coercive field and better fatigue properties.
Broadly speaking, the fabrication process includes the acts of providing an integrated circuit substrate comprising a thin film precursor and annealing the thin film precursor in the presence of an electrical field. The thin film precursor can comprise either a liquid coating, an amorphous precursor material to the ferroelectric material or a crystallized ferroelectric material. In one embodiment of the invention, voltage is applied to electrodes placed in close proximity to the precursor thin film. In a second embodiment of the invention, voltage is applied to electrodes in close proximity to the precursor thin film during the annealing process of the precursor thin film in a rapid thermal annealing apparatus. In a third embodiment of the invention, the integrated circuit substrate of the ferroelectric thin film material is deposited adjacent to a first electrode and then a second electrode is deposited onto the integrated circuit substrate, probes are connected directly to the electrodes of the integrated circuit substrate, and voltage is supplied to the electrodes via the probes. Preferably, in this third embodiment, the ferroelectric thin film material is preannealed prior to depositing the second electrode. Preferably, in all three embodiments, the electric field is sufficient to polarize the resultant ferroelectric material in the direction of the electrical field.
The invention provides a method of fabricating an integrated circuit, the method comprising the acts of: (a) providing an integrated circuit substrate; (b) depositing a precursor thin film on the integrated circuit substrate, the precursor thin film comprising either a ferroelectric material or a precursor to a ferroelectric material; (c) applying an electric field to the precursor thin film; (d) annealing the precursor thin film during the application of the electric field to form a solid ferroelectric thin film; and (e) completing the fabrication of the integrated circuit to include at least a portion of the solid ferroelectric thin film in an electrical component of the integrated circuit. Preferably, the solid ferroelectric thin film is an active material in the integrated circuit. Preferably, the act of applying an electric field comprises creating an electric field sufficient to align the polarization direction of the ferroelectric thin film with the electric field. Preferably, the act of annealing comprises heating the precursor thin film to a temperature above the crystallization temperature of the ferroelectric material. Preferably the act of annealing comprises heating the precursor thin film to a temperature above the Curie temperature of the ferroelectric material. In an alternative embodiment, the act of annealing comprises heating the precursor thin film to a temperature below the Curie temperature of the ferroelectric material. Preferably, the method further includes the act of cooling the ferroelectric thin film below the crystallization temperature of the ferroelectric material while applying the electric field. Preferably, said act of annealing comprises annealing in a furnace. Preferably, the act of annealing comprises annealing in a rapid thermal anneal apparatus. Preferably, the integrated circuit comprises a ferroelectric memory. Preferably, the ferroelectric memory comprises a ferroelectric capacitor. Preferably, the ferroelectric memory comprises a ferroelectric field effect transistor.
In another aspect, the invention provides a method of fabricating an integrated circuit, said method comprising: annealing the ferroelectric material or precursor to the ferroelectric material; and applying an electric field to the annealed ferroelectric material as it cools. Preferably, the electric field is applied until the film cools to the Curie temperature of the ferroelectric thin film or below. Preferably, the electric field is applied until the film cools to the crystallization temperature of the ferroelectric thin film or below. Preferably, the electric field is applied until the film cools to room temperature. Preferably, the act of applying comprises creating an electric field sufficient to align the polarization direction of the ferroelectric thin film with the electric field. Preferably, the act of annealing comprises heating the precursor thin film to a temperature above the crystallization temperature of the ferroelectric material. Preferably, the act of annealing comprises heating the precursor thin film to a temperature above the Curie temperature of the ferroelectric material. Alternatively, the act of annealing comprises heating said precursor thin film to a temperature below the Curie temperature of said ferroelectric material. Preferably, the precursor to the ferroelectric material is a solid. Preferably, the precursor to the ferroelectric material is a liquid. Preferably, the act of annealing comprises annealing in a furnace. Preferably, the act of annealing comprises annealing in a rapid thermal anneal apparatus.
Other features, objects and advantages will become apparent to those skilled in the art upon reading the detailed description below in combination with the accompanying drawings.